This invention relates generally to superheterodyne television tuners and particularly to such television tuners that are used to receive and process ultra high frequency (UHF) television signals.
Varactor diode tuners of the superheterodyne type are well-known in the art. In such tuners, the capacitance of a varactor diode in a tuned circuit is controlled by a tuning voltage. The resonant frequency of the tuned circuit may be altered by changing the tuning voltage. Varactor diodes are incorporated in a number of tunable circuits in the tuner for selecting a desired television signal from among the gamut of available television signals. A desired signal is selected or filtered by the tuner antenna stage and the selected signal is amplified. The amplified signal is further filtered by the interstage and then applied to a mixer stage. The selected signal is mixed or heterodyned, in the mixer stage, with a locally generated oscillator signal to produce an intermediate frequency (IF) output signal of fixed frequency. The oscillator frequency is usually a predetermined amount above the desired television signal carrier frequency and the IF signal is developed in the mixer by heterodyning the incoming television carrier signal with the local oscillator signal. An input circuit and interstage circuit, generally comprising tunable bandpass filters, are utilized to select the desired television signal from among the gamut of available television signals and to pass it to the mixer stage.
In superheterodyne tuners, there is an ever present concern with so-called image frequency signals. Image frequency signals are undesired signals that, when mixed with the oscillator signal, also produce a signal at the IF frequency. If the IF frequency is 45 MHz for example, an image frequency would exist at 90 MHz from the channel frequency. Assume a channel frequency of 375 MHz, an IF frequency of 45 MHz and an oscillator frequency of 420 MHz. A frequency of 465 MHz (90 MHz higher than the channel frequency), heterodyning with the oscillator signal would also generate a 45 MHz output signal that would directly interfere with the desired IF frequency signal. The 465 MHz signal is at an image frequency and is, of course, undesirable.
In prior art tuner designs, image frequencies are either ignored or generally rejected by a trap filter in the tuner interstage circuit. It is believed that in prior art tuner designs, an image frequency trap could not be employed in the tuner input circuit without significantly affecting the tuner signal-to-noise ratio and tuner input standing wave ratio.
In accordance with the invention, a tunable image frequency trap is employed in the tuner input circuit, i.e., the antenna circuit, of the tuner and consists of a tunable leg of a tunable parallel bandpass filter. The circuit of the invention utilizes discrete components which may be selected with precision and controlled over the tuning range to maintain the image frequency trap at the appropriate frequency.